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Our Peculiar Motion Away from the Local Void
The peculiar velocity of the Local Group of galaxies manifested in thecosmic microwave background dipole is found to decompose into threedominant components. The three components are clearly separated becausethey arise on distinct spatial scales and are fortuitously almostorthogonal in their influences. The nearest, which is distinguished by avelocity discontinuity at ~7 Mpc, arises from the evacuation of theLocal Void. We lie in the Local Sheet that bounds the void. Randommotions within the Local Sheet are small, and we advocate a referenceframe with respect to the Local Sheet in preference to the Local Group.Our Galaxy participates in the bulk motion of the Local Sheet away fromthe Local Void. The component of our motion on an intermediate scale isattributed to the Virgo Cluster and its surroundings, 17 Mpc away. Thethird and largest component is an attraction on scales larger than 3000km s-1 and centered near the direction of the CentaurusCluster. The amplitudes of the three components are 259, 185, and 455 kms-1, respectively, adding collectively to 631 kms-1 in the reference frame of the Local Sheet. Taking thenearby influences into account, particularly that of the Local Void,causes the residual attributed to large scales to align with observedconcentrations of distant galaxies and reduces somewhat the amplitude ofmotion attributed to their pull. Concerning the motion of ~260 kms-1 away from the Local Void, given the velocities expectedfrom gravitational instability theory in the standard cosmologicalparadigm, the distance to the center of the Local Void must be at least23 Mpc from our position. The Local Void is extremely large.
| The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem
We present Keck DEIMOS spectroscopy of stars in eight of the newlydiscovered ultra-faint dwarf galaxies around the Milky Way. We measurethe velocity dispersions of Canes Venatici I, Canes Venatici II, ComaBerenices, Hercules, Leo IV, Leo T, Ursa Major I, and Ursa Major II fromthe velocities of 18-214 stars in each galaxy and find dispersionsranging from 3.3 to 7.6 km s-1. The six galaxies withabsolute magnitudes MV<-4 are highly dark matterdominated, with mass-to-light ratios approaching 1000Msolar/Lsolar,V. For the fainter galaxies we findtentative evidence for tidal disruption. The measured velocitydispersions of the ultra-faint dwarfs are correlated with theirluminosities, indicating that a minimum mass for luminous galacticsystems may not yet have been reached. We also measure the metallicitiesof the observed stars and find that the new dwarfs have meanmetallicities of [Fe/H]=-2.0 to -2.3 these galaxies represent some ofthe most metal-poor stellar systems known. The six brightest of theultra-faint dwarfs extend the luminosity-metallicity relationshipfollowed by more luminous dwarfs by a factor of ~30 in luminosity. Wedetect metallicity spreads of up to 0.5 dex in several objects,suggesting multiple star formation epochs. UMa II and Com, despite theirexceptionally low luminosities, have higher metallicities that suggestthey may once have been much more massive. Having established the massesof the ultra-faint dwarfs, we re-examine the missing satellite problem.After correcting for the sky coverage of the Sloan Digital Sky Survey,we find that the ultra-faint dwarfs substantially alleviate thediscrepancy between the predicted and observed numbers of satellitesaround the Milky Way, but there are still a factor of ~4 too few dwarfgalaxies over a significant range of masses. We show that if galaxyformation in low-mass dark matter halos is strongly suppressed afterreionization, the simulated circular velocity function of CDM subhaloscan be brought into approximate agreement with the observed circularvelocity function of Milky Way satellite galaxies.
| The Minimum Amount of Stars a Galaxy Will Form
We present an analysis of the atomic hydrogen and stellar properties of38 late-type galaxies in the local universe covering a wide range of H Imass-to-light ratios (Script M/LB), stellar luminosities, andsurface brightnesses. Combining the results with those of four otherwell-studied dwarf galaxies known for their unusually large H Icontents, we identified an upper envelope for the Script M/LBas a function of galaxy luminosity. This implies an empirical relationbetween the minimum amount of stars a galaxy will form and its initialbaryonic mass. We also find that the star density systematicallydecreases with increasing Script M/LB, making the galaxiesoptically more elusive. While the stellar mass of a galaxy seems to beonly loosely connected to its baryonic mass, the latter quantity isstrongly linked to the galaxy's dynamical mass as it is observed in thebaryonic Tully-Fisher relation. We find that dwarf irregular galaxieswith generally high Script M/LB ratios follow the same trendas defined by lower Script M/LB giant galaxies but areunderluminous for their rotation velocity to follow the trend in astellar mass Tully-Fisher relation, suggesting that the baryonic mass ofthe dwarf galaxies is normal but they have failed to produce asufficient amount of stars. Finally, we present a three-dimensionalequivalent to the morphology-density relation which shows that highScript M/LB galaxies preferentially evolve and/or survive inlow-density environments. We conclude that an isolated galaxy with ashallow dark matter potential can retain a large portion of its baryonicmatter in the form of gas, only producing the minimum quantity of starsnecessary to maintain a stable gas disk.
| Hα survey of the local volume: Isolated southern galaxies
We present our Hα observations of 11 isolated southern galaxies:SDIG, PGC 51659, E 222-010, E 272-025, E 137-018, IC 4662, Sag DIG, IC5052, IC 5152, UGCA 438, and E 149-003, with distances from 1 to 7 Mpc.We have determined the total Hα fluxes from these galaxies. Thestar formation rates in these galaxies range from 10‑1(IC 4662) to 10‑4 M ȯyr‑1 (SDIG) and the gas depletion time at the observedstar formation rates lies within the range from 1/6 to 24 Hubble times H 0 ‑1 .
| Advanced Camera for Surveys Imaging of 25 Galaxies in Nearby Groups and in the Field
We present Hubble Space Telescope Advanced Camera for Surveys images andcolor-magnitude diagrams for 25 nearby galaxies with radial velocitiesVLG<500 km s-1. Distances are determined basedon the luminosities of stars at the tip of the red giant branch thatrange from 2 to 12 Mpc. Two of the galaxies, NGC 4163 and IC 4662, arefound to be the nearest known representatives of blue compact dwarfobjects. Using high-quality data on distances and radial velocities of110 nearby field galaxies, we derive their mean Hubble ratio to be 68 kms-1 Mpc-1 with a standard deviation of 15 kms-1 Mpc-1. Peculiar velocities of most of thegalaxies, Vpec=VLG-68D, follow a Gaussiandistribution with σv=63 km s-1 but with atail toward high negative values. Our data display the known correlationbetween peculiar velocity and galaxy elevation above the LocalSupercluster plane. The small observed fraction of galaxies with highpeculiar velocities, Vpec<-500 km s-1, may beunderstood as objects associated with nearby groups (Coma I, Eridanus)outside the local volume.
| Masses of the local group and of the M81 group estimated from distortions in the local velocity field
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
| The discovery of X-ray binaries in the Sculptor dwarf spheroidal galaxy
We report the results of a deep Chandra survey of the Sculptor dwarfspheroidal galaxy. We find five X-ray sources with LX of atleast 6 × 1033 ergs-1 with opticalcounterparts establishing them as members of Sculptor. These X-rayluminosities indicate that these sources are X-ray binaries, as no otherknown class of Galactic point sources can reach 0.5-8 keV luminositiesthis high. Finding these systems proves definitively that such objectscan exist in an old stellar population without stellar collisions. Threeof these objects have highly evolved optical counterparts (giants orhorizontal branch stars), as do three other sources whose X-rayluminosities are in the range which includes both quiescent low-massX-ray binaries and the brightest magnetic cataclysmic variables. Wepredict that large area surveys of the Milky Way should also turn uplarge numbers of quiescent X-ray binaries.
| Homogeneous Photometry. IV. On the Standard Sequence in the Globular Cluster NGC 2419
I present a new analysis of CCD-based BVRI broadband photometry for theglobular cluster NGC 2419, based on 340 CCD images either donated bycolleagues or retrieved from public archives. The calibrated resultshave been made available through my Web site. I compare the results ofmy analysis with those of an independent analysis of a subset of thesedata by Saha et al. (2005, PASP, 117, 37), who have found acolor-dependent discrepancy of up to 0.05 mag between their I-bandphotometry and mine for stars in this cluster. I conclude that a majorpart of the discrepancy appears to be associated with small shuttertiming errors (a few hundredths of a second) in the Mini-Mosaic (MIMO)camera on the WIYN 3.5 m telescope. Smaller contributions to the anomalylikely come from (1) a color-scale error with a maximum amplitude of~+/-0.02 mag in my secondary standard list as of 2004 September, and (2)statistical effects arising from the previous study's use of arelatively small number of standard-star observations to determine acomparatively large number of fitting parameters in the photometrictransformations.Based in part on observations obtained at the 3.5 m and 0.9 m WIYNTelescopes. The WIYN Observatory is a joint facility of the Universityof Wisconsin-Madison, Indiana University, Yale University, and theNational Optical Astronomy Observatory (NOAO).
| Imaging and photometry of nearby dwarf galaxies. II. Southern dwarfs
We carried out CCD photometry in the Johnson-Cousins B and R bands of 23dwarf galaxies: SDIG, ESO 410-17, KK11, ESO 245-05, KKs3, KK27, KK38,KK40, IC 4662, KK244, KK246, KK247, KK248, KK249, KK253, KK255, KK256,KK257, KK258, KK259, UGCA 438, ESO 347-17, and UGCA 442. Both isolatedgalaxies and members of the Sculptor group and the NGC 1313 group wereobserved. The galaxy sample is characterized by a median distance of 9.3Mpc, and median absolute magnitude of -14.8 mag. The central surfacebrightnesses are in the range from 22.2 to 24.4 mag arcsec-2in B.Based on observations obtained with CTIO 1.5-m telescope, which isoperated by the Association of Universities for Research in AstronomyInc. (AURA), under a cooperative agreement with the National ScienceFoundation as part of the National Optical Astronomy Observatories.Tables 1 and 2, complete Figs. 1 and 2 are only available in electronicform at http://www.edpsciences.org
| Light and Motion in the Local Volume
Using high-quality data on 149 galaxies within 10 Mpc, I find nocorrelation between luminosity and peculiar velocity at all. There is nounequivocal sign on scales of 1-2 Mpc of the expected gravitationaleffect of the brightest galaxies, in particular infall toward groups, orof infall toward the supergalactic plane on any scale. Either darkmatter is not distributed in the same way as luminous matter in thisregion, or peculiar velocities are not due to fluctuations in mass. Thesensitivity of peculiar velocity studies to the background model ishighlighted.
| The 1000 Brightest HIPASS Galaxies: H I Properties
We present the HIPASS Bright Galaxy Catalog (BGC), which contains the1000 H I brightest galaxies in the southern sky as obtained from the H IParkes All-Sky Survey (HIPASS). The selection of the brightest sourcesis based on their H I peak flux density (Speak>~116 mJy)as measured from the spatially integrated HIPASS spectrum. The derived HI masses range from ~107 to 4×1010Msolar. While the BGC (z<0.03) is complete inSpeak, only a subset of ~500 sources can be consideredcomplete in integrated H I flux density (FHI>~25 Jy kms-1). The HIPASS BGC contains a total of 158 new redshifts.These belong to 91 new sources for which no optical or infraredcounterparts have previously been cataloged, an additional 51 galaxiesfor which no redshifts were previously known, and 16 galaxies for whichthe cataloged optical velocities disagree. Of the 91 newly cataloged BGCsources, only four are definite H I clouds: while three are likelyMagellanic debris with velocities around 400 km s-1, one is atidal cloud associated with the NGC 2442 galaxy group. The remaining 87new BGC sources, the majority of which lie in the zone of avoidance,appear to be galaxies. We identified optical counterparts to all but oneof the 30 new galaxies at Galactic latitudes |b|>10deg.Therefore, the BGC yields no evidence for a population of``free-floating'' intergalactic H I clouds without associated opticalcounterparts. HIPASS provides a clear view of the local large-scalestructure. The dominant features in the sky distribution of the BGC arethe Supergalactic Plane and the Local Void. In addition, one can clearlysee the Centaurus Wall, which connects via the Hydra and Antlia Clustersto the Puppis Filament. Some previously hardly noticable galaxy groupsstand out quite distinctly in the H I sky distribution. Several newstructures, including some not behind the Milky Way, are seen for thefirst time.
| A Catalog of Neighboring Galaxies
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
| A test for the origin of quasar redshifts
It is commonly accepted that quasar redshifts have a cosmologicalcharacter and that most of the quasars are at Gpc distances. However,there are some cases where several quasars with completely differentredshifts and a nearby active galaxy are aligned in a certain way oroccupy a very small patch on the sky, which is claimed by some authorsto be unlikely to happen by chance. Is there a small subset of quasarswith non-cosmological redshifts? For quasars apparently associated withgalaxies, we consider two scenarios for the origin of their redshift:(i) a change in the scale factor of the whole Universe (standard,cosmological scenario), and (ii) a velocity-induced Doppler shift of thespectrum of a nearby object (local, ejection scenario). We argue for asimple astrometric test which can distinguish between these two sourcesof quasar redshifts by constraining their proper motions.We give the predictions for the maximum possible proper motions of aquasar for the cosmological and local scenarios of the origin of theirredshifts. We apply these theoretical results to the Bukhmastovacatalogue, which contains more than 8000 close quasi-stellarobject-galaxy associations. In the standard interpretation of quasarredshifts, their typical proper motions are a fraction of μas, andbeyond the reach of planned astrometric missions such as GAIA and SIM.On the other hand, the quasars ejected from local active galactic nucleiat velocities close to the speed of light would have proper motions 5-6orders of magnitude larger, which would easily be measurable with futureastrometric missions, or even in some cases with HST, VLT and Kecktelescope. The distributions of proper motions for the cosmological andlocal scenarios are very well separated. Moreover, the divisioncorresponds nicely to the expected accuracy from GAIA and SIM.
| Halo Substructure and the Power Spectrum
We present a semianalytic model to investigate the merger history,destruction rate, and survival probability of substructure inhierarchically formed dark matter halos and use it to study thesubstructure content of halos as a function of input primordial powerspectrum. For a standard cold dark matter ``concordance'' cosmology(ΛCDM n=1, σ8=0.95) we successfully reproducethe subhalo velocity function and radial distribution profile seen inN-body simulations and determine that the rate of merging and disruptionpeaks ~10-12 Gyr in the past for Milky Way-like halos, while survivingsubstructures are typically accreted within the last ~0-8 Gyr. Weexplore power spectra with normalizations and spectral ``tilts''spanning the ranges σ8~=1-0.65 and n~=1-0.8, andinclude a ``running-index'' model with dn/dlnk=-0.03 similar to thebest-fit model discussed in the first-year Wilkinson MicrowaveAnisotropy Probe (WMAP) report. We investigate spectra with truncatedsmall-scale power, including a broken-scale inflation model and threewarm dark matter cases with mW=0.75-3.0 keV. We find that themass fraction in substructure is relatively insensitive to the tilt andoverall normalization of the primordial power spectrum. All of theCDM-type models yield projected substructure mass fractions that areconsistent with, but on the low side, of published estimates from stronglens systems: f9=0.4%-1.5% (64th percentile) for subhalossmaller than 109 Msolar within projected cylindersof radius r<10 kpc. Truncated models produce significantly smallerfractions, f9=0.02%-0.2% for mW~=1 keV, and aredisfavored by lensing estimates. This suggests that lensing and similarprobes can provide a robust test of the CDM paradigm and a powerfulconstraint on broken-scale inflation/warm particle masses, includingmasses larger than the ~1 keV upper limits of previous studies. Wecompare our predicted subhalo velocity functions with the dwarfsatellite population of the Milky Way. Assuming that dwarfs haveisotropic velocity dispersions, we find that the standard n=1 modeloverpredicts the number of Milky Way satellites atVmax<~35 km s-1, as expected. Models with lesssmall-scale power do better because subhalos are less concentrated andthe mapping between observed velocity dispersion and haloVmax is significantly altered. The running-index model, or afixed tilt with σ8~0.75, can account for the localdwarfs without the need for differential feedback (forVmax>~20 km s-1) however, these comparisonsdepend sensitively on the assumption of isotropic velocities insatellite galaxies.
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| Distances to nearby galaxies in Sculptor
We present an analysis of Hubble Space Telescope/WFPC2 images of ninenearby galaxies in Sculptor. We derive their distances from theluminosity of the tip of the red giant branch stars with a typicalaccuracy of ~ 12%. Their distances are 4.21 Mpc (Sc 22), 4.92 Mpc (DDO226), 3.94 Mpc (NGC 253), 3.40 Mpc (KDG 2), 3.34 Mpc (DDO 6), 3.42 Mpc(ESO 540-030), 4.43 Mpc (ESO 245-05), 4.27 Mpc (UGCA 442), and 3.91 Mpc(NGC 7793). The galaxies are concentrated in several spatially separatedloose groups around NGC 300, NGC 253, and NGC 7793. The Sculptor galaxycomplex together with the CVn I cloud and the Local Group form a 10 Mpcfilament, apparently driven by the free Hubble flow.Based on observations made with the NASA/ESA Hubble Space Telescope. TheSpace Telescope Science Institute is operated by the Association ofUniversities for Research in Astronomy, Inc. under NASA contract NAS5-26555.
| The little galaxy that could: kinematics of Camelopardalis B
We present deep, high velocity resolution (~1.6 km s-1) GiantMeterwave Radio Telescope HI 21 cm synthesis images, as well as opticalbroad band images, for the faint (MB~-10.9) dwarf irregulargalaxy Camelopardalis B. We find that the HI in the galaxy has a regularvelocity field, consistent with rotational motion. Further, the impliedkinematical major axis is well aligned with the major axis of both theHI flux distribution as well as that of the optical emission. CamelopardalisB is the faintest known galaxy with such relatively well behavedkinematics. From the HI velocity field we derive a rotation curve forthe galaxy. The rotation curve can be measured out to galacto-centricdistances >4 times the optical scale length. The peak (inclinationcorrected) rotation velocity vo is only ~7 kms-1-the high velocity resolution of our observations washence critical to measuring the rotation curve. Further, the peakrotational velocity is comparable to the random velocity σ of thegas, i.e. vo/σ~1. This makes it crucial to correct theobserved rotation velocities for random motions before trying to use thekinematics to construct mass models for the galaxy. After applying thiscorrection we find a corrected peak rotation velocity of ~20 kms-1. On fitting mass models to the corrected rotation curvewe find a good fit for a constant density halo with a density ofρ0~12 Msolar pc-3. This density iswell determined, i.e. it has a very weak dependence on the assumed massto light ratio of the stellar disk. We also find that the correctedrotation curve cannot be fit with an NFW halo regardless of the assumedmass to light ratio. Finally we compile from the literature a sample ofgalaxies (ranging from normal spirals to faint dwarfs) with rotationcurves obtained from HI synthesis observations. The complete samplecovers a luminosity range of ~12 magnitudes. From this sample we find:(i) that Camelopardalis B lies on the Tully-Fisher relation defined bythese galaxies, provided we use the corrected rotation velocity, and(ii) a weak trend for increasing halo central density with decreasinggalaxy size. Such a trend is expected in hierarchical models of haloformation.
| Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.
| Interstellar Medium Abundances in Sculptor Group Dwarf Irregular Galaxies
Using the Cerro Tololo Inter-American Observatory 4 m telescope, we haveobtained optical spectra of H II regions in five Sculptor group dwarfirregular galaxies. We derive oxygen, nitrogen, and sulfur abundancesfrom the H II region spectra. Oxygen abundances are derived via threedifferent methods (the ``direct'' method, the empirical method guided byphotoionization modeling of McGaugh [published in 1991], and the purelyempirical method of Pilyugin, published in 2000) and are compared.Significant systematic differences are found between the three methods,and we suggest that a recalibration of the empirical abundance scale isrequired. Until differences between these three methods are betterunderstood, the issue of the degree of uniformity of the interstellarmedium abundances in a dwarf galaxy cannot be properly addressed. TheN/O ratio for the metal-poor dI ESO 473-G24 of log(N/O)=-1.43+/-0.03lies well above the plateau of log(N/O)=-1.60+/-0.02 found by Izotov& Thuan for a collection of metal-poor, blue compact galaxies. Thisshows that not all galaxies with 12+log(O/H)<=7.6 have identicalelemental abundance ratios, and this implies that the Izotov & Thuanscenario for low-metallicity galaxies is not universal. Measurements ofthe H II regions in NGC 625 yield log(N/O)~-1.25. Assuming N productionby intermediate-mass stars, this relatively high N/O ratio may beindicative of a long quiescent period prior to the recent active burstof star formation. The oxygen abundances in the Sculptor group dI's arein good agreement with the relationship between metallicity andluminosity observed in the Local Group dI's. Taken together, theobservations show a better relationship between metallicity andluminosity than between metallicity and galaxy central surfacebrightness. The Sculptor group dI's, in general, lie closer to thesimple closed-box model evolutionary path than the Local Group dI's. Thehigher gas contents, lower average star formation rates, and closerresemblance to closed-box evolution could all be indicative of evolutionin a relatively low density environment.
| Star Formation in Sculptor Group Dwarf Irregular Galaxies and the Nature of ``Transition'' Galaxies
We present new Hα narrowband imaging of the H II regions in eightSculptor group dwarf irregular (dI) galaxies. The Hα luminositiesof the detected H II regions range from some of the faintest detected inextragalactic H II regions (~1035 ergs s-1 in SC24) to some of the most luminous (~1040 ergs s-1in NGC 625). The total Hα luminosities are converted into currentstar formation rates (SFRs). Comparing the Sculptor group dI's to theLocal Group dI's, we find that the Sculptor group dI's have, on average,lower values of SFR when normalized to either galaxy luminosity or gasmass (although there is considerable overlap between the two samples).The range for both the Sculptor group and Local Group samples is largewhen compared with that seen for the sample of gas-rich, quiescent, lowsurface brightness (LSB) dI's from van Zee et al. (published in 1997)and the sample of isolated dI's from van Zee (from 2000 and 2001). Thisis probably best understood as a selection effect since the nearby groupsamples have a much larger fraction of extremely low luminosity galaxiesand the smaller galaxies are much more liable to large relativevariations in current SFRs. The Sculptor group and LSB samples are verysimilar with regard to mean values of both τgas andτform, and the Local Group and isolated dI samples arealso similar to each other in these two quantities. Currently, theSculptor group lacks dI galaxies with elevated normalized current SFRsas high as the Local Group dI's IC 10 and GR 8. The properties of``transition'' (dSph/dIrr) galaxies in Sculptor and the Local Group arealso compared and found to be similar. The transition galaxies aretypically among the lowest luminosities of the gas-rich dwarf galaxies.Relative to the dwarf irregular galaxies, the transition galaxies arefound preferentially nearer to spiral galaxies and are found nearer tothe center of the mass distribution in the local cloud. While most ofthese systems are consistent with normal dI galaxies, exhibitingtemporarily interrupted star formation, the observed density-morphologyrelationship (which is weaker than that observed for the dwarfspheroidal galaxies) indicates that environmental processes such as``tidal stirring'' may play a role in causing their lower SFRs.
| Local galaxy flows within 5 Mpc
We present Hubble Space Telescope/WFPC2 images of sixteen dwarf galaxiesas part of our snapshot survey of nearby galaxy candidates. We derivetheir distances from the luminosity of the tip of the red giant branchstars with a typical accuracy of ~ 12%. The resulting distances are4.26 Mpc (KKH 5), 4.74 Mpc (KK 16), 4.72 Mpc (KK 17), 4.66 Mpc (ESO115-021), 4.43 Mpc (KKH 18), 3.98 Mpc (KK 27), 4.61 Mpc (KKH 34), 4.99Mpc (KK 54), 4.23 Mpc (ESO 490-017), 4.90 Mpc (FG 202), 5.22 Mpc (UGC3755), 5.18 Mpc (UGC 3974), 4.51 Mpc (KK 65), 5.49 Mpc (UGC 4115), 3.78Mpc (NGC 2915), and 5.27 Mpc (NGC 6503). Based on distances and radialvelocities of 156 nearby galaxies, we plot the local velocity-distancerelation, which has a slope of H0 = 73 km s-1Mpc-1 and a radial velocity dispersion of 85 kms-1. When members of the M81 and Cen A groups are removed,and distance errors are taken into account, the radial velocitydispersion drops to sigmav = 41 km s-1. The localHubble flow within 5 Mpc exhibits a significant anisotropy, with twoinfall peculiar velocity regions directed towards the Supergalacticpoles. However, two observed regions of outflow peculiar velocity,situated on the Supergalactic equator, are far away ( ~ 50degr ) fromthe Virgo/anti-Virgo direction, which disagrees with a sphericallysymmetric Virgo-centric flow. About 63% of galaxies within 5 Mpc belongto known compact and loose groups. Apart from them, we found six newprobable groups, consisting entirely of dwarf galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope. TheSpace Telescope Science Institute is operated by the Association ofUniversities for Research in Astronomy, Inc. under NASA contract NAS5-26555.}\fnmsep\thanks{Table 2, and Figs. 1 and 2, are only availablein electronic form at http://www.edpsciences.org
| The very local Hubble flow
We present Hubble Space Telescope/WFPC2 images of eighteen galaxiessituated in the vicinity of the Local Group (LG) as part of an ongoingsnapshot survey of nearby galaxies. Their distances derived from themagnitude of the tip of the red giant branch are 1.92±0.10 Mpc(ESO 294-010), 3.06±0.37 (NGC 404), 3.15±0.32 (UGCA 105),1.36±0.07 (Sex B), 1.33±0.08 (NGC 3109), 2.64±0.21(UGC 6817), 2.86±0.14 (KDG 90), 2.27±0.19 (IC 3104),2.54±0.17 (UGC 7577), 2.56±0.15 (UGC 8508),3.01±0.29 (UGC 8651), 2.61±0.16 (KKH 86), 2.79±0.26(UGC 9240), 1.11±0.07 (SagDIG), 0.94±0.04 (DDO 210),2.07±0.18 (IC 5152), 2.23±0.15 (UGCA 438), and2.45±0.13 (KKH 98). Based on the velocity-distance data for 36nearest galaxies around the LG, we find the radius of the zero-velocitysurface of the LG to be R0 = (0.94±0.10) Mpc, whichyields a total mass MLG = (1.3±0.3) ×1012 Msolar. The galaxy distribution around the LGreveals a Local Minivoid which does not contain any galaxy brighter thanMV=-11 mag within a volume of ~100 Mpc3. The localHubble flow seems to be very cold, having a one-dimensional mean randommotion of ~30 km s-1. The best-fit value of the local Hubbleparameter is 73±15 km s-1 Mpc-1. Theluminosity function for the nearby field galaxies is far less steep thanone for members of the nearest groups. Figure 2 is only available in theelectronic form at http://www.edpsciences.org Based on observations madewith the NASA/ESA Hubble Space Telescope. The Space Telescope ScienceInstitute is operated by the Association of Universities for Research inAstronomy, Inc. under NASA contract NAS 5-26555.
| HIPASS High-Velocity Clouds: Properties of the Compact and Extended Populations
A catalog of southern anomalous-velocity H I clouds at decl. <+2° is presented. This catalog is based on data from the H I ParkesAll-Sky Survey (HIPASS) reprocessed with the MINMED5 procedure andsearched with a new high-velocity cloud-finding algorithm. The improvedsensitivity (5 σ: ΔTB= 0.04 K), resolution(15.5′), and velocity range (-500 kms-1
| Local Field of Galaxy Velocities
A sample of 145 galaxies having radial velocities relative to thecentroid of the Local Group V LG D H ij , with principal values of81:62:48 in km/sec·Mpc, which have a standard error of 4km/sec·Mpc. The minor axis of the Hubble ellipsoid is orientedalmost along the polar axis of the Local Supercluster, while the majoraxis forms an angle = (29 ± 5)° with the direction toward thecenter of the Virgo Cluster. Such a configuration of thepeculiar-velocity field shows unsatisfactory agreement with the model ofa spherically symmetric flow of galaxies toward the Virgo Cluster.Rotation of the Local Supercluster may be one reason for thisdifference. The peculiar velocities of galaxies within a volume with D V= 74 km/sec, a considerable part of which is due to the virial motionsof galaxies in groups and to distance errors. For field galaxies,located in a layer of 1 < D < 3 Mpc around the Local Group, theradial-velocity dispersion does not exceed 25 km/sec. Thevelocitydistance relation, constructed from the 20 closestgalaxies around the Local Group with D < 3 Mpc and with errorsσ(D) < 0.2 Mpc, exhibits the expected effect of gravitationaldeceleration. Using the estimate of R 0 = (0.96 ± 0.05) Mpc forthe observed radius of the zero-velocity sphere, we determined the totalmass of the Local Group to be (1.2 ± 0.2)·1012 M ȯ,which agrees well with the sum of the virial masses of the subgroups ofgalaxies around the Local Group and M31. The ratio of the Local Group'stotal mass (within R 0) to its luminosity is M/L = (23 ± 4) Mȯ/L ȯ, which does not require the existence of supermassivedark halos around our Galaxy and M31.
| The Various Kinematics of Dwarf Irregular Galaxies in Nearby Groups and Their Dark Matter Distributions
Eight dwarf irregular galaxies, in the two nearby groups of galaxiesSculptor and Centaurus A (at 2.5 Mpc and 3.5 Mpc), have been imaged inneutral hydrogen (H I) with the Australia Telescope and the Very LargeArray. These galaxies have absolute magnitudes ranging fromMB=-15.7 to -11.3. Yet they are mostly rotationallysupported, with maximum velocities going from 19 to 67 kms-1. Multicomponent mass models have been fitted to therotation curves to investigate the properties of their dark matter halosand the scaling laws of dark matter halo parameters. Dwarf galaxieshave, on average, a higher dark to luminous mass ratio, as well ashigher dark halo central densities than spiral galaxies. They have alarger dispersion of their dark matter properties both in terms of theirtotal dark matter amount and of their dark halo parameters, compared tospiral galaxies. It is therefore very difficult to predict a dwarfgalaxy rotation curve shape based only on its optical properties. Dwarfsare not well fitted by cold dark matter (CDM) halos of the type proposedby Navarro, Frenk, & White, even for ΛCDM models withΩ0 as low as 0.3. For two of our dwarfs we also haveHα rotation curves confirming the H I velocities, so thediscrepancy with the CDM models cannot be attributed to beam-smearingeffects.
| Hubble Space Telescope Photometry of the Dwarf Spheroidal Galaxy ESO 410-G005
We present Hubble Space Telescope WFPC2 imaging of the nearby lowsurface brightness dwarf spheroidal galaxy ESO 410-G005, which has beenresolved into stars for the first time. The resulting color-magnitudediagram for about 2500 stars shows a red giant branch with a tip atI=22.4+/-0.15 mag, which yields a distance of DMW=1.9+/-0.2Mpc. ESO 410-G005 is found to be metal-poor with a mean metallicity of<[Fe/H]>=(-1.8+/-0.4) dex estimated from its red giant branch.Upper asymptotic giant branch stars appear to be present near the centerof the galaxy, indicative of a substantial, centrally concentratedintermediate-age population, unless these objects are artifacts ofcrowding. Previous studies did not detect ESO 410-G005 in Hα or inH I. Based on our distance estimate, ESO 410-G005 is a probable memberof the Sculptor group of galaxies. Its linear separation from thenearest spiral, NGC 55, is 230 kpc on the sky. The deprojectedseparation ranges from 340 to 615 kpc depending on the assumed distanceof NGC 55. The deprojected distance from the Sculptor group spiral NGC300 is (385+/-200) kpc. ESO 410-G005 appears to be a relatively isolateddSph within the Sculptor group. Its absolute magnitude,MV,0=-12.1+/-0.2 mag, its central surface brightness,μV,0=22.7+/-0.1 mag arcsec-2, and its meanmetallicity, [Fe/H]=(-1.8+/-0.4) dex follow the trend observed for dwarfgalaxies in the Local Group. Based on observations made with theNASA/ESA Hubble Space Telescope. The Space Telescope Science Instituteis operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555.
| The elliptical galaxy formerly known as the Local Group: merging the globular cluster systems
Prompted by a new catalogue of M31 globular clusters, we have collectedtogether individual metallicity values for globular clusters in theLocal Group. Although we briefly describe the globular cluster systemsof the individual Local Group galaxies, the main thrust of our paper isto examine the collective properties. In this way we are simulating thedissipationless merger of the Local Group, into presumably an ellipticalgalaxy. Such a merger is dominated by the Milky Way and M31, whichappear to be fairly typical examples of globular cluster systems ofspiral galaxies. The Local Group `Elliptical' has about 700 +/- 125globular clusters, with a luminosity function resembling the `universal'one. The metallicity distribution has peaks at [Fe/H] ~ -1.55 and -0.64with a metal-poor to metal-rich ratio of 2.5:1. The specific frequencyof the Local Group Elliptical is initially about 1 but rises to about 3,when the young stellar populations fade and the galaxy resembles an oldelliptical. The metallicity distribution and stellar populationcorrected specific frequency are similar to that of some known earlytype galaxies. Based on our results, we briefly speculate on the originof globular cluster systems in galaxies.
| A New Local Group Galaxy in Cetus
We report the discovery of a previously uncataloged Local Group galaxyin the constellation Cetus. Faintly visible on UKST survey plates, ithas a smooth, diffuse appearance and appears to be a dwarf spheroidal oftype dE3.5. A color-magnitude diagram in V, V-I shows a clear giantbranch but no sign of recent star formation. From the position of thetip of the giant branch, we derive a reddening-corrected distancemodulus of 24.45+/-0.15 and a metallicity of -1.9+/-0.2. With an impliedheliocentric distance of 775+/-50 kpc, and a corresponding Local Groupbarycentric distance of 615 kpc, the Cetus dwarf lies well within theboundaries of the Local Group, and although it currently lacks a radialvelocity measurement, it is undoubtedly a member of the Local Group. Thenearest Local Group galaxies are WLM and IC 1613 at angular separationsof 7.3d and 16.3d and roughly 175 and 230 kpc total distance,respectively. Although the Cetus dwarf is unlikely to be directlyassociated with any other Local Group galaxy, it does lie in the generaldirection of the extension of the Local Group toward the Sculptor Group.
| HI properties of nearby galaxies from a volume-limited sample
We consider global HI and optical properties of about three hundrednearby galaxies with V_0 < 500 km s(-1) . The majority of them haveindividual photometric distance estimates. The galaxy sample parametersshow some known and some new correlations implying a meaningful dynamicexplanation: 1) In the whole range of diameters, 1 - 40 Kpc, the galaxystandard diameter and rotational velocity follows a nearly linearTully-Fisher relation, lg A25~(0.99+/-0.06)lg V_m. 2) The HImass-to-luminosity ratio and the HI mass-to-``total" mass (inside thestandard optical diameter) ratio increase systematically from giantgalaxies towards dwarfs, reaching maximum values 5 ;M_ȯ/L_ȯand 3, respectively. 3) For all the Local Volume galaxies their totalmass-to-luminosity ratio lies within a range of [0.2-16]M_ȯ/L_ȯ with a median of 3.0 ;M_ȯ/L_ȯ. TheM25/L ratio decreases slightly from giant towards dwarfgalaxies. 4) The M_HI/L and M25/L ratios for the samplegalaxies correlate with their mean optical surface brightness, which maybe caused by star formation activity in the galaxies. 5) The M_HI/L andM25/L ratios are practically independent of the local massdensity of surrounding galaxies within the range of densities of aboutsix orders of magnitude. 6) For the LV galaxies their HI mass andangular momentum follow a nearly linear relation: lgM_HI~(0.99+/-0.04)lg (V_m* A25), expected for rotatinggaseous disks being near the threshold of gravitational instability,favourable for active star formation. Table in the Appendix is availableonly in electronic form at the CDS via anonymous ftp 130.79.128.5 orhttp//cdsweb.u-strasbg.fr/Abstract.html
| Dust-to-Gas Ratio and Metal Abundance in Dwarf Galaxies
We have compared the metallicity (represented by oxygen abundance), Xo,and the dust-to-gas ratio, D , in a sample of dwarf galaxies. For dwarfirregulars (dIrrs) we find a good correlation between the twoquantities, with a power-law index of 0.52 +/- 0.25. Blue compact dwarf(BCD) galaxies do not show such a good correlation; in addition, boththe dust-to-gas ratio and the metallicity tend to be higher than fordIrrs. We have then developed a simple but physical analytical model forthe above relation. Comparing the model results with the data, weconclude that (i) for low values of D , the D-Xo relation isquasi-linear, whereas for higher values, the curve strongly deviatesfrom the linear behavior, implying that the commonly used power-lawapproximation is very poor; (ii) the deviation from the linear behaviordepends critically on the parameter chi , the "differential" massoutflow rate from the galaxy in units of the star formation rate, psi ;(iii) the shape of the D-Xo curve does not depend on psi ,but only on chi ; however, the specific location of a given galaxy onthe curve does depend on psi ; and (iv) the BCD metallicity segregationis the result of a higher psi , together with a significant differentialmass outflow. Thus, the lack of correlation can be produced by largelydifferent star formation rates and values of chi in these objects.
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